Fractionation of polymers

ABSTRACT

Process of obtaining a high molecular weight fraction from a polar polymer which comprises the steps of first, forming a solution of the polar polymer in a solvent which is also a solvent for a selected coacervating substance, then mixing with the solution a coacervating substance having at least two proton donating or proton receiving groups which are opposite the polarity of the polar polymer in an amount sufficient to crosslink molecules of said polar polymer of a molecular weight above a selected range at the solution temperature, thereby forming coacervated high molecular weight polymer solids, and then separating the high molecular weight solids from the solution. The coacervated polymer may then be dissolved and the coacervating substance removed by precipitation.

This is a continuation of application Ser. No. 45,580 filed June 4,1979, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to processes for purifying polymers, and moreparticularly, this invention relates to processes for separating highmolecular weight fractions from polymers containing a range of molecularweights.

2. Description of the Prior Art

As polymer technology is extended into more product areas, there is anincreasing need for purification of polymeric substances. The nature ofthe impurities found in polymers depends on the origin of the polymers.In the case of synthetic polymers, the impurities are usually associatedwith the method of synthesis. In the case of natural polymers, theimpurities are associated with the natural source of the polymer and themethod of extraction or separation from that source.

In addition to simple removal of nonpolymeric impurities, it isfrequently important to separate polymeric substances from relativelylow or relatively high molecular weight species of the same polymericsubstance. The reasons for this need to separate specific narrowmolecular weight fractions having improved physical and chemicalproperties from a mixture are well known in the art but will becomefurther apparent later in this disclosure.

There are many processes reported which are either expensive to operateor suitable only for laboratory preparation of pure, specific molecularweight fractions of polymeric substances. The most important of theseprocesses include precipitation from solution by addition of anonsolvent liquid, chromatographic separation, sedimentation techniques,diffusion techniques, and ultrafiltration using porous membranes.

U.S. Pat. No. 2,945,016 relates to a process for separation of polymerscomprising adding an open chain hydrocarbon to a solution of normallysolid polymer in a cyclic hydrocarbon solvent, thereby causing theformation of a polymer-rich phase and a solvent-rich phase, andrecovering at least one of said phases.

SUMMARY OF THE INVENTION

The present invention provides a process for obtaining a high molecularweight fraction from a polymer which comprises forming a solution of thepolar polymer in a solvent which is also a solvent for a selectedcoacervating substance, mixing with the solution a coacervatingsubstance having at least two proton donating or proton receiving groupswhich are opposite the polarity of said polar polymer in an amountsufficient to cross link molecules of said polar polymer of a molecularweight above a selected range at the solvent temperature, therebyforming coacervated high molecular weight polymer solids, and separatingthe high molecular weight solids from the solution. The separated highmolecular weight solids may then be washed and separated from thecoacervating substance. Use of this process provides a simple, precise,and convenient way of extracting high molecular weight fractions fromthe polymer.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, polar polymers are fractionatedaccording to molecular weight, so that the polymer fraction within aselected molecular weight range may be isolated. The process comprisesforming a solution of the polar polymer in a solvent which is also asolvent for a selected coacervating substance, and then coacervating thehigher molecular weight polymer fraction by adding a selectedcoacervating substance to form a complex. The precipitated orcoacervated polymer complexes may then be washed and regenerated.

Polar polymers which may be used in the process of the invention includenatural and synthetic polyelectrolytes, polymeric acids and polymericbases. For example, polar polymers include those polymers containingcarbonyl, sulfate, sulfonate, phosphate, and phosphite groups, and saltsof the groups. Also, polar polymers include those polymers containingunreacted acidic phenolic groups, polymers containing salts of phenolicgroups, polymers containing weak acidic groups or salts of these groupssuch as, for example, the enolic form of diketones and the pendantinorganic radicals such as arsenic.

Basic polar polymers include polymers containing unreacted organic basicradicals or salts of the radicals such as primary, secondary, andtertiary aliphatic amine groups, i.e., ##STR1## wherein R₁ and R₂ can beH, alkyl or aryl having from 1 to 20 carbon atoms, and cyclic aminegroups, i.e., ##STR2## wherein R₃ can be H or alkyl having from 1 to 20carbon atoms, and R₄ and R₅ are members of an alicyclic ring structure.

Polar polymers also include those polymers derived from vinylsubstituted heterocyclic ring structures containing at least 4% nitrogenbased on the total weight of the heterocyclic radical and wherein thetotal weight of such groups in the polymer are from 1 to 37 weightpercent. Some preferred examples of such structures are: ##STR3##wherein R₇ is any organic radical or radicals having a total molecularweight of less than 300.

Such compositions as homopolymers and copolymers containing themonomeric constituents vinylpyridine, derivatives of vinylpyridine,N,N-dialkyl ether esters and amides of acrylic and methacrylic acids,vinylcarbazole, allyl amine, diallylamine, ethylenimine, and derivativesof ethylenimine are included. Also included are either synthetic ornatural polymers which are rendered basic by chemical reaction such ascellulose propionate morpholinobutyrate, reduction product ofpolyacrylonitriles, polystyrene and derivatives of polystyrene. Alsoincluded are the acid salts and quaternary ammonium derivatives of basicamine containing polymers and quaternary derivatives of sulfide groupscontaining polymers.

Such compositions as homopolymers and copolymers containing themonomeric constituents acrylic acid, methacrylic acid, maleic acid,fumaric acid, itaconic acid, and ethylenically unsaturated monomerscontaining sulfate, phosphate, sulfonate, phosphonate, and phosphitegroups are included. Also included are natural and synthetic polymerswhich contain unreacted acidic functionality obtained from chemicalreaction starting with a neutral polymer.

Polymers containing both acidic and basic functionality can befractionated by the process of the invention. For instance, polymerscontaining both amino groups and carboxylic acid groups such as thecopolymer of N,N-diethylaminoethyl acrylate and acrylic acid may beseparated from monomeric impurities and/or high or low molecular weightfractions. The invention is of particular value in the purification ofnatural proteins where coacervation and separation is performed at ornear the isoelectric pH.

In the broadest sense, solvents or mixtures of solvents used in thisinvention are capable of dissolving the polar polymers and selectedcoacervating substances, but allow precipitation or insolubilization ofhigh molecular weight portions of polar polymers with coacervatingsubstances in the solvent media.

Many solvents are useful in this invention. Useful solvent types includewater, alcohols, e.g.; methanol, ethanol, isopropanol, n-propanol,n-butanol, isobutanol, octanol, and ethylene glycol; acetones, e.g.;acetone, methyl ethyl ketone, diethyl ketone, and methyl isobutylketone; esters, e.g.; ethyl acetate, isopropyl acetate, butyl acetate;organic acids, e.g.; acetic acid, formic acid, propionic acid, andisobutyric acid; ethers, e.g.; methyl ethyl ether, diethyl ether,1,4-dioxane; mineral acids and bases, e.g.; dilute aqueous hydrochloricacid, dilute aqueous sulfuric acid, dilute phosphoric acid, and dilutealkali solutions; and aromatic solvents, e.g.; benzene, xylene.

Some other useful solvents include nitromethane, dimethyl formamide,tetrahydrofuran, pyridine, chloroform, trichloroethylene, methylenechloride, ethylenedichloride, and buffered aqueous systems.

Mixtures of two or more solvents may be used. Mixtures of solvents andnonsolvents for a particular polymer may also be used. The purpose ofusing mixtures of solvents or mixtures of solvents and nonsolvents for aparticular polymer is to adjust the polarity of the solvent systems. Thedesired polarity of the solvent is dependent on the particular polymerbeing purified. The polymer-solvent relationship is usually adjusted sothat, although the polymer is fully dissolved, only small changes arerequired to precipitate the polymer. The precise adjustment requiredwill be obvious to one skilled in the art of dissolving polymers.

Nonsolvents include n-pentane, n-hexane, n-heptane, cyclohexane,toluene, hydrogenated naphthalenes, water, carbon tetrachloride, diethylether, petroleum ether and ethyl acetate.

The term coacervating substance is used herein to mean acids or baseswhich have at least two acidic or two basic groups opposite to thepolarity of the polymer, and are capable of reacting with the dissolvedpolymer and wherein the reaction product is at least partiallyprecipitated from solution in the selected solvent.

The polyfunctional acids or bases used in this invention have thegeneral formula:

    (X.sub.1).sub.m --Q--(X.sub.2).sub.n

where X₁ and X₂ represent proton-donating or proton-receiving groups, Qis the inorganic or organic moiety of the molecule, m and n are themultiplicity of the particular acid or base group. The totalmultiplicity, n+m, must be equal to or greater than two.

A wide variety of organic acids or bases may be employed in thisinvention. Organic acids in which there are at least two acid groupssuch as carboxylic, sulfur and phosphorus containing acid moieties maybe employed in this invention. Organic bases in which there are at leasttwo basic groups such as primary, secondary, tertiary, quaternaryamines, heterocyclic amines, aromatic amines, may also be employed inthis invention.

Examples of organic acids useful as coacervating substances includeoxalic acid, malonic acid, succinic acid, adipic acid, sebacic acid,dimer acid, tartaric acid, phthalic acid, terephthalic acid, trimesicacid, trimellitic acid, 1,2,4,5-benzenetetracarboxylic acid,polysulfonic acids, and pyrophosphates.

Examples of organic bases useful as coacervating substances includeethylenediamine, unsymmetrical dimethylethylenediamine,N,N,N',N'-tetraethylenediamine, trimethylenediamine,hexamethylenediamine, phenylenediamine, imidazole, pyrimidine, purine,nicotine and 2,2'-bipyridine.

Examples of amphoteric organic compounds useful as coacervatingsubstances include amino acids, polyamino acids, proteins andethylenediaminetetracetic acid.

Also included as coacervating substances are polymers containing acidicand basic groups. Some typical coacervating agents of this type arepolymers and copolymers containing acrylic acid, methacrylic acid,maleic acid, fumaric acid, itaconic acid, vinyl substituted pyridines,vinyl quinolines, dialkyl substituted aminoethyl esters, and amides ofacrylic and methacrylic acid, vinyl carbazole, and modified naturalpolymers such as cellulose propionate morpholinobutyrate and carboxycellulose.

The process of this invention comprises first dissolving the selectedpolar polymer in the solvent. Solubility characteristics are well knownin the art and are described in technical literature, for example, in"Textbook of Quantitative Inorganic Analysis" by Kolthoff and Sandell(The Macmillan Company, New York, 1948). Temperatures at or near roomtemperature are often preferred. The polymer and solvent are preferablyagitated to speed up the dissolving process. Next, a selected quantityof coacervating substance, which is also soluble in the selectedsolvent, is added to the solution to coacervate the higher molecularweight fraction of the dissolved polymer. The quantity of coacervatingsubstance used to coacervate the selected fraction is determined byroutine experimentation. The final amount used depends on the objective,i.e., the smaller the quantity of coacervating substance used, thesmaller the range of higher molecular weight polymer solidified. Thepolymer-solvent ratio is preferably adjusted so that, although thepolymer is fully dissolved, only small changes are required tocoacervate the polymer. The higher molecular weight fraction solidifiesprior to lower molecular weight fractions as the coacervating substanceis added.

The higher molecular weight fraction of polymer and coacervatingsubstance is separated from the solution by conventional techniques suchas filtering or centrifuging. Following separation, the fraction is thenpurified by washing and separation of the polymer from the coacervatingsubstance. The polymer is separated from the coacervating substance bydissolving the coacervated polymer in a suitable solvent andprecipitating the coacervating substance. Suitable solvents andprecipitating agents for the particular materials are well known bythose skilled in the art. The precipitated coacervating substance isthen removed, such as by filtering or centrifuging, to leave the polymerremaining in solution.

The following examples are submitted for a better understanding of theinvention.

EXAMPLE 1

Ten grams of poly(2-methyl-5-vinylpyridine/styrene) are dissolved in 200ml acetone and 0.3 g succinic acid in acetone as coacervating substanceis added. The solution is vigorously agitated. The supernate whichcontains low molecular weight species is removed by the centrifugationmethod. The test is repeated using 0.4 and 0.6 g succinic acid. Thenumber average molecular weights of the polymers in the supernatesobtained by boiling point elevation method are shown as follows:

    ______________________________________                                                              No. Average                                                      Succinic Acid Added                                                                        Molecular Wt.                                           ______________________________________                                        Supernate 1                                                                              0.3 g          2258                                                Supernate 2                                                                              0.4 g          859                                                 Supernate 3                                                                              0.6 g          839                                                 ______________________________________                                    

These results clearly indicate that separation of low molecular weightpolymers is achieved by adjusting the amount of coacervating substanceadded to the polymer solutions.

EXAMPLE 2

Ten grams poly(2-methyl-5-vinylpyridine/styrene) are dissolved in 200 mlacetone. Three-tenths gram succinic acid in acetone is added to thepolymer solution with vigorous agitation at ˜40° C. After the cloudymixture is cooled, the supernate and polymer coacervates are separatedby centrifugation.

The coacervate is then washed with 200 ml acetone three times withagitation, heating, cooling, and centrifugation steps as described inExample 1 above. The purified coacervate is removed and dissolved in 200ml 95/5 acetone/water. One gram Ca(OH)₂ is then added to precipitate thesuccinic acid in the solution. After the solution is cooled, the polymersolution is separated from the precipitate by centrifugation.

It is found that the supernate has lower molecular weight than theparent polymer, and the purified polymer has higher molecular weightthan the original polymer. The low molecular weight species (M.W. ≦1000)in the purified polymer is determined to be less than 10 ppm byultramembrane filtration followed by UV analysis of the vinyl pyridinemoiety. The original polymer contains 1600 ppm low molecular weightspecies by the same analytical measurement.

EXAMPLE 3

Ten grams polymer purified by the process described in Example 2 isdissolved in 200 ml acetone. Three-tenths gram succinic acid is added toprecipitate the polymer. After removing the supernate, the polymercoacervate is then dispersed in boiling acetone solution which contains1% water. The solution is cooled to ˜5° C. to allow the precipitation ofthe polymer succinic acid complex. The polymer-succinic acid complex isthen washed two more times with acetone which contains 1% water. Othersteps to regenerate purified polymer are the same as described inExample 2. The final purified sample exhibits higher molecular weightand a narrower molecular weight distribution than Example 2.

EXAMPLE 4

Ten grams of 80/20 methylmethacrylic acid/methyl methacrylate polymerare dissolved in acetone. Ethylenediamine, 0.4 g, in methanol is addedto the polymer solution. The precipitate is separated from the supernateby centrifugation. The precipitate is then washed with acetone/methanolsolution 4 times as in the processes described in Example 2 and 3.Finally the precipitate is dissolved in methanol. A small amount ofdilute HCl is added to precipitate the diamine. The polymer purified hasa higher molecular weight and a narrower molecular weight distributionthan the starting material. The first supernate solution after removingthe precipitate is condensed and analyzed for molecular weightdistribution. It contains a major portion of the low molecular weightspecies from the original polymer solution.

EXAMPLE 5

Ten grams of 2-methyl-5-vinylpyridine/methylmethacrylic acid polymer isdissolved in acetone. 0.2 g low molecular weight polyacrylic acid(I.V.=0.01) in acetone is added to the polymer solution. The supernateis analyzed for molecular weight distribution. It exhibits much lowermolecular weight distribution and contains about 90% of the lowmolecular weight species. The precipitate is further washed three timeswith acetone/methanol solution. The purified polymer complex is thendispersed in ethanol/water. Calcium hydroxide is added to precipitatepolyacrylic acid. The purified polymer exhibits higher average molecularweight than the original polymer. The low molecular weight species inthe purified polymer is lower than the detection limit by U.V. analysisof the pyridine moiety.

EXAMPLE 6

Poly(4-vinylpyridine) is fractionated by coacervation techniques using amethanol and water solvent system and trimesic acid as a coacervatingsubstance. The polymer may be dissolved in the methanol/water system atratios of 100/0 to 42/58. The polymer may be precipitated atmethanol/water ratios of from 42/58 to 0/100. The non-overlapping ofsolubility of the polymer and trimesic acid allows the proper selectionof methanol/water ratios which may enhance the precipitation of thetrimesic acid. The separation of polymer from trimesic acid can then beachieved. For example, 2.5 g poly(4-vinylpyridine) is dissolved in 10 mlof methanol. Water is added until the cloud point is reached. The volumeof water to reach this point is 13.6 ml. Thus, the minimummethanol/water ratio for the polymer to stay in solution is 42/58.

Unless otherwise specified, all parts, percentages, ratios, etc., are ona weight basis.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:
 1. The process of obtaining a high molecular weight fraction from a polymer which comprises the steps of(a) forming a solution of the polar polymer in a solvent which is also a solvent for a selected coacervating substance, (b) mixing with said solution a coacervating polymer having at least two proton donating or proton receiving groups which are opposite the polarity of said polar polymer in an amount sufficient to crosslink molecules of said polar polymer of a molecular weight above a selected range at the solution temperature, said coacervating substance being a polyfunctional acid or base having the formula

    (X.sub.1).sub.m --Q--(X.sub.2).sub.n

wherein X₁ and X₂ represent proton-donating or proton-receiving groups, Q is an organic moiety, and m and n are the multiplicity of the acidic or basic group, thereby forming coacervated high molecular weight polymer solids, and (c) separating the high molecular weight solids from the solution.
 2. The process of claim 1 wherein the ratio of polar polymer to solvent is adjusted such that the polymer is completely dissolved, but the solution is near the saturation point.
 3. The process of claim 1 wherein the high molecular weight solids separated in step (c) are dissolved, the coacervating substance is precipitated and separated from the polymer in solution. 